When a common diode is in series to a load in flow direction on a DC circuit it will drop voltage by .6 volts. If the diode is inserted in the opposite direction no current will flow.

On an AC circuit a single diode will drop flow in one direction and conduct in the other direction with a .6 volt drop. The resulting power will be about half the effective AC current and become half wave pulsed DC current. Thus a diode in series to a light bulb would dim it in half. This is an easy way to dim lights in a passenger car for example,a diode inserted in series to the bulbs would effectively reduce 18 volts to 9 volts and consume half the power. Alternating diode polarity between cars would keep would balance draw on the sine wave and keep load symmetric. That is point one towards the bulb and the next car away from the bulb. Half waving vibrator type PW accessories makes them work better because of the imbalanced sine wave.

On an AC circuit when 2 parallel diodes are inserted in series in opposite polarity voltage will drop by .6 volts when current is pulled through them. So a voltage dropping circuit can be made using diodes or bridge rectifiers. A bridge rectifier is really 4 diodes. When the + and - are connected the taps become paired diodes in opposite polarity. This is useful in accessory circuits where individual accessories can be adjusted voltage wise with only 1 transformer tap. Connections can be made using euro style barrier strips. Here is a drawing of using diodes and rectifiers as a voltage dropper.

Another use for the voltage dropper is to slow a conventional engine and keep it from making jack rabbit starts. 4 paired 6 amp diodes for example inserted in series from the reverse board to the motor will drop voltage by 2.4 volts. So on the first throttle setting the motor would get 2.4 volts less but the reverse board,lights and smoke unit would get the full voltage. Typically the diode string would use 3-10 watts of power.

Dont know if you can make this out in the middle of the pic,there is a row of bridge rectifiers used as an AC voltage dropper. Relays come on in stages gradually accelerating the train in .6 volt intervals

Here is a more clear picture.It is part of a collision avoidance block system for super streets. The voltage dropper is made from 25 amp bridge rectifiers which provides adjustable voltage taps to vary car speeds to a super streets loop. 6 bridge rectifiers drop up to about 8 volts in .6 volt increments. They are connected to a Euro strip for easy adjustable connections. Used with timers and relays below and to the right, it randomly varies connection taps so that the cars do not all run at the same speed all the time. This makes super streets look more realistic.